Grand Unified Theories predict magnetic monopoles — inflation dilutes them to unobservable density
Monopole density
ρ/ρ_c ≫ 1
E-folds N
0
Scale factor
1.0
Universe fate
Collapse!
The Monopole Problem (Preskill 1979): At the GUT phase transition, the Higgs field chooses a vacuum independently in each causal horizon — creating one 't Hooft-Polyakov monopole per horizon volume. These monopoles are massive (~10¹⁶ GeV), and their predicted density would overclose the Universe (ρ_mono ≫ ρ_critical), causing immediate recollapse. Inflation (Guth 1981) solves this: N≥60 e-folds of exponential expansion dilutes monopoles to <1 per observable Universe.